Mineralogical study of some Antarctic monomict eucrites including Yamato-74356: A unique rock containing recrystallized clastic matrix

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We studied the thermal and impact history of four Yamato Antarctic monomict eucrites, Yamato (Y)-74356,Y-791186,Y-792510,and Y-82037,by mineralogical techniques. Although the mineral chemistry of Y-74356 is typical of monomict eucrites, it contains a recrystallized clastic matrix similar to that of some lunar granulitic breccias. Y-74356 experienced shock brecciation during a major thermal event. In contrast, Y-791186 and Y-792510 have pyroxenes of the highest metamorphic degree but the clastic matrices are weakly recrystallized, i. e., these two rocks were not extensively metamorphosed after breccia assembly. This indicates that the process that caused brecciation is distinct from the major thermal event. Y-82037 is a coarse-grained, unbrecciated ordinary eucrite, indicating that it was not affected by the brecciation event, except for the presence of a granoblastic portion in part of the pyroxene crystals. We deduce that the thermal metamorphism that formed ordinary eucrites took place on a longer time scale and at a lower temperature than that which formed some lunar granulitic breccias. The precursors of these ordinary eucrites have suffered from smaller scale brecciation events than those of the lunar granulitic breccias (metamorphic polymict breccias). It is unlikely that there were comparably large cratering events on the small HED parent body to cause the strong thermal metamorphism. The thermal events may have taken place at a very early stage of the evolution of the primary crust of the HED parent body.
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Mineralogical study of some Antarctic monomict eucrites including Yamato-74356: A unique rock containing recrystallized clastic matrix

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